Polyurethane varnishes are superior in a number of characteristics to a great many other varnishes, including reactive varnishes. In particular they are distinguished by high resistance to chemicals, especially acids, by high elasticity combined with great hardness in some cases, and by good resistance to wear.
The prototype of the polyurethane varnishes is the two-component varnish, consisting of synthetic resins containing hydroxyl groups (e.g., polyesters, polyethers, polyacrylates) and an NCO terminal adduct on the basis of diisocyanates. Such a two-component varnish by its very nature has the disadvantage of a limited pot life ranging, as a rule, from 6 to 20 hours.
Another approach to the preparation of polyurethane varnishes is to react resins containing hydroxyl groups (polyesters, polyethers, polyacrylates) with diisocyanates in an OH:NCO ratio of 1:2 or the like, or in any event with an excess of NCO groups. The resulting prepolymers containing NCO groups then set under the influence of atmospheric humidity to form films. To all intents and purposes this is a one-component system, but one which must be protected against the action of moisture. This implies that special precautions must be taken in the manufacture, pigmentation, etc., of these varnishes.
In a number of technical applications, as for example in roller varnishing, where the insensitivity of the system to surrounding influences, such as air and humidity, and constancy of viscosity over long periods of time are important, such systems cannot be used. It has hitherto been necessary to get along without the good characteristics of the polyurethane varnishes in such applications.
It is known that there are a number of substances which react with isocyanates to form chemically neutral substances, which can in turn be cleaved at elevated temperature to form isocyanates again. These blocked products would be ideally suited for the composition of the types of varnishes desired for this kind of application. A number of these products, however, must be rejected because they tend to yellow from the heat during the setting process. Furthermore, one must distinguish between the aromatic isocyanates and the aliphatic or cycloaliphatic isocyanates, since only the latter group of substances assures resistance to light and to weather simultaneously.
Now, if .epsilon.-caprolactam is used as the blocking agent, i.e., as the reagent, in order to transform isocyanates to a form that is non-reactive at lower temperatures, no difficulty is encountered in achieving a setting process that is free of yellowing. This cannot be done in practice, of course, since .epsilon.-caprolatam blocked isocyanates are incompatible in solvents commonly used in varnishes.